If the DDR3 cannot even pass the stress test at a low frequency, say 200 MHz, it is more likely to be a hardware problem. Then, it would be a good starting point to check the DDR3 connections. In addition,you should check the power supply for the DDR3, i.MX6 MMDC and DDR3 I/O to see if voltage is valid and ripple is not too large.

It looks like the device must pass 200 MHz, and should we check our device hardware in this case?